Cellulose acylates are used as a support of polymer film contained in photosensitive materials, such as photographic films, owing to advantageous characteristics, for example rigidity, non-flammability, and the like. A typical example of cellulose acylate is cellulose triacetate (TAC) having an average acetylation degree of 58.0-62.5%. Also, the polymer film of the cellulose triacetate (TAC) is used as a protection film of a polarizing element, or an optical compensation film (view angle enlarging film or the like), any of those being incorporated in a liquid crystal display (LCD) panel. This is effective because of optically utilizing the highly isotropic property of the polymer film.
A solution casting is a widely used method of producing polymer film of the cellulose triacetate (TAC). The solution casting is capable of producing a base film of photosensitive material or transparent sheet on a liquid crystal display panel. A solution casting of producing polymer film of cellulose triacetate (TAC) is typically suggested in JIII Journal of Technical Disclosure Monthly (Japan Hatsumei Kyokai, Kokai Giho), No. 2001-1745. At first, cellulose triacetate (TAC) is dissolved in a mixed solvent of which a main component is dichloromethane, to prepare dope or polymer solution. After this, the dope is cast on a support, to form cast film. Examples of supports are a casting support band and rotatable drum-shaped support. When the cast film dries to have a self-supporting property, a self-supporting cast film is stripped by a stripping roller. The cast film is sufficiently dried and cooled, and wound as a roll of the polymer film.
Generally in solution casting, surface quality of polymer film greatly depends upon a condition of stripping a cast film from the cast film as a self-supporting cast film. It is known as disclosed in JP-A 2003-071863 to set the surface temperature of the rotatable roll in a range of 6-15 deg. C. for cooling, and set time of contact of the cast film on the rotatable roll in a range of 0.5-20 seconds. This is for the purpose of facilitating the stripping of the cast film and also preventing a cloudy state on the polymer film. Also, dew point of a position of the stripping is controlled at a level equal to or less than 7 deg. C., so as to ensure prevention of the cloudy state.
To raise productivity of polymer films, speed of solution casting should be higher and higher. In contrast, high quality and high performance of polymer film as an optical element is desired by recent trends of enlargement of display panels and heightening of precision of displayed images. There occurs a problem of unavailability of polymer film having the intended optical performance in the case of stripping conditioned according to JP-A 2003-071863.
In view of the foregoing problems, an object of the present invention is to provide a solution casting process capable of producing a polymer film with high optical performance.